1. Field of the Invention
The invention is related to NMR logging techniques in a downhole environment in petrophysical testing. In particular, the invention quantifies and corrects for contamination effects from contaminant materials located in a borehole.
2. Description of the Related Art
Nuclear Magnetic Resonance (NMR) has uses in many areas, including the fields of medicine, chemistry, non-destructive testing, and in well logging in the oil exploration industry. In the well logging industry, NMR wireline logging or measurement-while-drilling (MWD) instruments are useful for collecting information on earth formation properties and for characterizing reservoir fluids. NMR is used in determining properties such as porosity of the formation, permeability, the movable fluid volume (BVM), the clay bound volume (CBW) and bulk volume irreducible (BVI), as well as other formation and reservoir fluid properties.
In a typical NMR device used in logging, a permanent magnet produces a static magnetic field and establishes a direction of orientation for nuclear magnetic moments in the vicinity of the borehole. An RF field is applied in the plane perpendicular to the static magnetic field. Typically in the art, the static field B0 is a function of distance from the tool. Thus, at a given applied frequency, the NMR resonance condition must be satisfied, wherein
                    ω        =                              γ            ⁢                                                  ⁢                          B              0                                            2            ⁢                                                  ⁢            π                                              (        1        )            where ω is the frequency of the RF field, and γ is the gyromagnetic ratio. Nuclei that are influenced by the applied RF field typically lie within a certain volume, named the sensitive volume. For a selected operating frequency, the location and size of the sensitive volume are determined by the magnetic field intensity, the field gradient and the effective bandwidth of the pulse. In multi-frequency logging, a discrete number of closely spaced and substantially non-overlapping sensitive volumes can be obtained. The union of these sensitive volumes is defined as the region of examination of a given tool with a given acquisition method.
In centralized tools, the region of examination is a cylindrical shell which is coaxial with the permanent magnet, although other spatial arrangements can be used. Since the region of examination typically lies close to the surface of the borehole cavity, a perfectly coaxial alignment of the tool and borehole wall, in which the borehole wall is circular and smooth, would yield optimal values of echo signals. Often though, geometric anomalies concerning the logging tool and the surface of the borehole will result in portions of the region of examination lying inside the borehole cavity rather than inside the rock formation. As one example of possible anomalies, the tool can be off-axis with the borehole and additionally can be lying against one side of the borehole, revealing a portion of the region of examination to the borehole cavity. In another example, the borehole might have an elliptical cross-section rather than a circular one. In yet a third possibility, there can be a significant amount of washout, where certain segments of the wall have separated and fallen away, leaving a cavity to one side of the borehole.
Current NMR logging or MWD instruments investigate formations only up to a few inches from the borehole wall. Therefore, irregularities in borehole geometry (such as an elliptical borehole shape) and/or formation damage (such as a cave-in) can affect these shallow NMR measurements. Such adverse effects are referred to as borehole contamination. The severity of the borehole contamination problem depends on the location of the NMR sensitive volume, which is often referred to as the depth of investigation, or DOI. For NMR instruments that can acquire data using multiple frequencies, the DOI is frequency-dependent and can be positively determined once the tool's magnetic configuration and its operation frequencies are known. Thus, the severity of borehole contamination to the NMR data is also frequency-dependent.
Drilling mud is typically used to facilitate drilling, and therefore yields a constant presence within the borehole. Typically, drilling mud is either oil-based (including synthetic oil-based), water-based, or glycol-based and hence has a large number of hydrogen nuclei. Due to the large number of hydrogen nuclei, the mud is a strong source of contamination in NMR spin echo signals, and the contamination signals can be greater than the desired signals obtained from the rock formation. To avoid receiving signals from within the borehole fluid, it is clearly desirable to have the region of examination contained within the rock formation and outside the borehole. When some portion of the region of examination lies within the borehole, NMR signals are received from material that is contained inside the borehole, usually drilling mud.
Drilling mud typically contains 80% or more of fluids. This is much higher than the fluid content of the surrounding rock formation. Contamination of borehole signals in NMR by mud signals spoils all critical petrophysical estimates including porosity, permeability, and T2 distribution. U.S. Pat. No. 6,603,310, to Georgi et al., having the same assignee as the present invention, discusses a method for correcting downhole NMR data contaminated by signals from borehole fluids. Correction may be made using either a reference porosity obtained from an independent source (such as density log). Alternatively, the amount of contamination is estimated with the aid of a standoff measuring device to determine the fractional volume of the region of investigation of the NMR tool that lies within the borehole. The characteristics of the borehole fluids are either known or are measured within the borehole at a depth where the entire volume of investigation lies within the borehole or from laboratory characterization of the mud sample.
In general, there are two categories of borehole mud: water-based mud (WBM) and oil-based mud (OBM). For NMR signals, glycol based mud has a behavior that is intermediate to WBM and OBM, but for simplicity, we include glycol based mud as a WBM in the discussion of the present invention. Both OBM and WBM contain clay particles, additives, emulsifiers, water, and in the case of OBM, base oil. The NMR signal in borehole mud for wells drilled with WBM is mainly from the water. In the case of OBM, the signal is mainly due to water and oil. These fluid molecules are surrounded by a large amount of clay particles, smaller in size but abundant in surface areas, which effectively shorten the relaxation time of mud fluids. Oil in the OBM may also wet the clay surfaces. Emulsifiers further homogenize the OBM, which is necessary to make base oil and clay particles well mixed and not spontaneously separated.
The method of Georgi provides one of the unique parameters from NMR logging: the bulk volume irreducible (BVI). The reliability of the method of Georgi is largely dependent on the correctness of the reference porosity, which is usually the density porosity. However, because density porosity is also a shallow DOI measurement, density porosity itself can be contaminated by borehole rugosity. If both NMR and reference porosities are contaminated, the comparison of the two porosity estimates may be inconclusive and may not properly identify the extent of borehole contamination problem. Moreover, even if the reference porosity is correct, it may be acquired with different vertical resolutions and from different logging passes. To match the vertical resolution and depths of the two different logs may introduce additional error. Therefore, it is indeed an advantage if a borehole contamination indicator is derived from NMR log data alone and the problem is corrected without the aid of a reference porosity.
A robust method for indicating borehole contamination benefits from being sensitive to the borehole contamination and insensitive to the field gradient changes associated with DOI. Also, insensitivity to random noise and processing artifacts are necessary characteristics of a robust indicator method. There is a need for detecting and correcting borehole contamination effects in NMR measurement techniques in multi-volume NMR logging. The present invention fulfills this need.